Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



William, Depending on the depth of the area, you could use Top'n Bond in multiple layers. Your will need to let each layer dry for about an hour and then apply the next layer. The project will need to be finished within 24 hours. If you cannot put the next layer on within 24 hours then you will have to wait 28 days before you can continue. The surface would have to be clean and free of any loose material, dust, debris, paints, sealers, and oils. Apply the material while the surface is damp.
- Lee-Technical Service
Monday, August 29, 2016 at 3:11 PM

Bob, yes it can be done but it would be considered a free float over the existing slab. You have to also keep in mind that you do not want to level the area; it has to have a 1/4" per foot pitch away from the house for proper drainage. If you level it then you will end up with standing water against the house and it will rot you seals. With a thickness 2" and greater you would need to use a concrete mix and apply a bonding agent. The surface would have to be clean and free of any loose material, dust, debris, paints, sealers, and oils. Apply the Sakrete Bonder & Fortifier and allow it to dry. Then mix it in the concrete mix as well replacing 50% of the water with Bonder & Fortifier. Pour the High Strength Concrete Mix in place. Make sure that you honor any expansion joints in the existing slab. If there are no expansion joints then I would recommend adding at least 3 to this slab leaving you with 3 lengths at around 8 feet per section. You will also need to incorporate expansion joints at all abutted walls.
- Lee-Technical Service
Monday, August 29, 2016 at 2:55 PM

I have two needs: 1. New concrete has a low spot (slope) right in front of the doorway. Need to top and re slope (3'X3') 2. Concrete drive (old) is low in a dead end area and cannot drain. Can I re slope with a topping (10' X 10')
- William J. Burfitt
Sunday, August 28, 2016 at 11:16 AM

Vera, the Flo-Coat Concrete Resurfacer could be used to correct the surface. However, you will have to wait 28 days from the time that the slab was poured before you could resurface it.
- Lee-Technical Service
Thursday, August 25, 2016 at 9:40 AM

I have an existing slab on a remodel porch. The area is 13' by 30' to level it would require the slab to be covered with 2" on the house side and 3.5 inches on the outside wall side. Is it possible to do this with an over poor product.
- Bob in Tampa
Wednesday, August 24, 2016 at 9:00 PM

We had a concrete slab for a basketball court put in and the surface is very rough and pitted. The work was done 5 days ago. The contractor has said he would come back to put on another coat, and I'm afraid he doesn't really know what to do. Which product would you recommend and tips on applying? And should he wait 28 days before recoating? Thanks
- Vera
Tuesday, August 23, 2016 at 8:03 PM

Ken, yes the Top-N-Bond can be used. Make sure that the wall is clean and free of any loose dust, debris, paints, and sealers. Then dampen the wall and apply the Top N Bond. If the material wants to sag away from the wall then you may have to do the job in multiple lifts. So put on a thin layer and allow to dry for an hour and then install the next layer.
- Lee-Technical Service
Wednesday, August 17, 2016 at 4:45 PM

Can Top'n Bond be used to repair a spalling concreted stem wall. I blasted all the loose concrete off. I will be placing back from a feather edge to approximately 3/8" thick. All work is above grade
- Ken
Wednesday, August 17, 2016 at 10:50 AM

Bill, the Fast Setting Cement Patcher will work fine for that application. It can be placed from 1/4" up to 2" and has great abrasion resistance.
- Lee-Technical Service
Wednesday, August 17, 2016 at 9:58 AM

I am topping existing concrete slab to create a level surface in an area approx. 30' x 40' with low spots varying from 1/2" to 1 1/2" then back up to 1/2". There will be new rolling lockers for golf club storage on top so we must be level but not a large amount of weight. What is the best product to use in this large area?
- Bill in Naples
Tuesday, August 16, 2016 at 4:43 PM



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